1. Field of the Invention
The present invention relates to magnetic amorphous alloys and to a method of annealing these alloys in a magnetic field. The present invention is also directed to amorphous magnetostrictive alloys for use in a magnetomechanical electronic article surveillance system. The present invention furthermore is directed to a magnetomechanical electronic article surveillance system employing such a marker, as well as to a method for making the amorphous magnetostrictive alloy and a method for making the marker.
2. Description of the Prior Art
It is well known from Chikazumi, Physics of Magnetism (Robert E. Krieger Publishing Company, Malbar, Fla.) chapter 17, p. 359 ff. (1964), for example, that most ferromagnetic alloys exhibit a uniaxial anisotropy when they are heat-treated in a magnetic field whereby the induced magnetic easy axis is parallel to the direction of the annealing field or, more generally, parallel to the domain magnetization during annealing. The aforementioned Chikazumi text gives an example for the magnetization curve of a permalloy (crystalline Fe-Ni alloy) sample measured in a direction perpendicular to the induced magnetic easy axis. Chikazumi notes that in this case the magnetization takes place through a rotation of each magnetic domain giving rise to a linearly ascending magnetization curve.
Luborsky et al., "Magnetic Annealing of Amorphous Alloys", IEEE Trans. on Magnetics MAG-11, p. 1644-1649 (1975) give an early example for magnetic field annealing of amorphous alloys. They transversely field-annealed amorphous Fe.sub.40 Ni.sub.40 P.sub.14 B.sub.6 alloy strips in a magnetic field of 4 kOe which was oriented across the ribbon width, i.e. perpendicular to the ribbon axis and in the ribbon plane. After a 2 hrs. treatment at 325.degree. C. and subsequent cooling of 50 deg/min and 0.1 deg/min, for example, they found a hysteresis loop with virtually vanishing remanence and linear dependence of the magnetization versus the applied field up to ferromagnetic saturation which occurs when the applied field equals or exceeds the induced anisotropy field. The authors attributed their observation to the fact that the magnetic field annealing induces a magnetic easy axis transverse to the ribbon direction and that upon applying a magnetic field the magnetization changes by rotation out of this easy axis.
Actually amorphous metals are particularly sensitive to magnetic field annealing owing to the absence of magneto-crystalline anisotropy as a consequence of their glassy non-periodic structure. Amorphous metals can be prepared in the form of thin ribbons by rapidly quenching from the melt which allows a wide range of compositions. Alloys for practical use are basically composed of Fe, Co and/or Ni with an addition of about 15-30 at % of Si and B (Ohnuma et al., "Low Coercivity and Zero Magnetostriction of Amorphous Fe--Co--Ni System Alloys" Phys. Status Solidi (a) vol. 44, pp. K151 (1977)) which is necessary for glass formation. The virtually unlimited miscibility of the transition metals in the amorphous state yields a large versatility of magnetic properties. According to Luborsky et al., "Magnetic Anneal Anisotropy in Amorphous Alloys", IEEE Trans. on Magnetics MAG-13, p. 953-956 (1977) and Fujimori "Magnetic Anisotropy" in F. E. Luborsky (ed) Amorphous Metallic Alloys, Butterworths, London, pp. 300-316 (1983) alloy compositions with more than one metal species are particularly susceptible to the magnetic field anneal treatment. Thus, the magnitude of the induced anisotropy K.sub.u can be varied by choice of the alloy composition as well as by appropriate choice of the annealing temperature and time to range from a few J/m.sup.3 up to about 1 kJ/m.sup.3. Accordingly the anisotropy field which is given by H.sub.K =2 K.sub.u /J.sub.s (cf. Luborsky et al., "Magnetic Annealing of Amorphous Alloys", IEEE Trans. on Magnetics MAG-11, p. 1644-1649 (1975); J.sub.s is the saturation magnetization) and which, for a transversely field-annealed material, defines the field up to which the magnetization varies linearly with the applied field before reaching saturation, can be varied from values well below 1 Oe up to values of approximately H.sub.k.apprxeq.25 Oe.
The linear characteristics of the hysteresis loop and the low eddy current losses both associated with transversely field-annealed amorphous alloys are useful in a variety of applications such as transformer cores, for example (cf. Herzer et al, "Recent Developments in Soft Magnetic Materials", Physica Scripta vol T24, p 22-28 (1988)). Another field of application where transversely annealed amorphous alloys are particularly useful makes use of their magnetoelastic properties which is described in more detail in the following.
Becker et al., Ferromagnetismus (Springer, Berlin), ch. 5, pp. 336 (1939) or Bozorth, Ferromagnetism (d. van Nostrand Company, Princeton, N.J.) ch. 13, p 684 ff (1951) explain in their textbooks that the magnetostriction associated with rotation of the magnetization vector is responsible for the fact that in ferromagnetic materials Young's modulus changes with the applied magnetic field, which is usually referred to as the .DELTA.E effect.
Consequently U.S. Pat. No. 3,820,040and Berry et al. "Magnetic annealing and Directional Ordering of an Amorphous Ferromagnetic Alloy", Physical Reviews Letters, vol. 34, p. 102-1025 (1975) realized that an amorphous Fe-based alloy, when transversely field annealed, exhibits a .DELTA.E effect two orders of magnitude larger than for crystalline iron. They attributed this striking difference to the lack of magnetocrystalline anisotropy in the amorphous alloy, which allows a much greater response to the applied stress by magnetization rotation. They also demonstrated that annealing in a longitudinal field largely suppresses the .DELTA.E effect since in this condition the domain orientations are not susceptible to stress-induced rotation. In the Berry 1975 et al. article it is recognized that the enhanced .DELTA.E effect in amorphous metals provides a useful means to achieve control of the vibrational frequency of an electromechanical oscillator with the help of an applied magnetic field.
The possibility to control the vibrational frequency by an applied magnetic field was found to be particularly useful in European Application 0 093 281 for markers for use in electronic article surveillance (EAS). The magnetic field for this purpose is produced by a magnetized ferromagnetic strip (bias magnet) disposed adjacent to the magnetoelastic resonator, with the strip and resonator being contained in a marker or tag housing. The change in effective magnetic permeability of the marker at the resonance frequency provides the marker with signal identity. This signal identity can be removed by-chanding the resonant frequency by means of the applied field. Thus the marker can, for example, be deactivated by degaussing the bias magnet, which removes the applied magnetic field, and thus changes the resonant frequency appreciably. Such systems originally (cf. European Application 0 093 281, and Application PCT WO 90/03652) used markers made of amorphous ribbons in the "as prepared" state which also can exhibit an appreciable .DELTA.E effect owing to uniaxial anisotropies associated with production-inherent mechanical stresses.
U.S. Pat. No. 5,469,140 discloses that the application of transverse field annealed amorphous magnetomechanical elements in electronic article surveillance systems removes a number of deficiencies associated with the markers of the prior art which use as prepared amorphous material. In an example, this patent describes a linear behavior of the hysteresis loop up to an applied field of at least about 10 Oe. This linear behavior associated with the transverse field annealing avoids the generation of harmonics which can produce undesirable alarms in other types of EAS systems (i.e., harmonic systems). Such interference with harmonic systems actually is a severe problem with the aforementioned magneto-elastic markers of the prior art, due the non-linear hysteresis loop typical associated with the as prepared state of amorphous alloys, since it is this non-linear behavior which (undesirably) triggers an alarm in a harmonic EAS system. This patent further teaches that heat treatment in a magnetic field significantly improves the consistency in terms of the resonant frequency of the magnetostrictive strips. A further advantage of such annealed resonators is their higher resonant amplitude. This patent also teaches that a preferred material is an Fe--Co alloy which contains at least about 30 at % Co, whereas earlier materials of the prior art such as Fe.sub.40 Ni.sub.38 Mo.sub.3 B.sub.18, disclosed in the aforementioned PCT Application WO 90/03652 are unsuitable in pulse-field magnetomechanical EAS systems since annealing such materials undesirably reduces the ring down period of the signal. In German Gebrauchsmuster G 94 12 456.6 the present inventor recognized that a long ring-down time can be achieved by choosing an alloy composition which reveals a relatively high induced magnetic anisotropy and that, therefore, such alloys are particularly suited for magnetoelastic markers in article surveillance systems. Herzer teaches that the desired high ring-down times can be also achieved at lower Co-contents down to about 12 at % if, starting from a Fe--Co-based alloy, up to about 50% of the Fe and/or Co is substituted by Ni. The need for a linear loop with relatively high anisotropy and the benefit of alloying Ni in order to reduce the Co-content for such magnetoelastic markers was later on reconfirmed by the disclosure of U.S. Pat. No. 5,628,840.
The field annealing in the aforementioned examples was done across the ribbon width i.e. the magnetic field direction was oriented perpendicular to the ribbon axis and in the plane of the ribbon surface. This technique will be referred to herein, and is known in the art, as transverse field-annealing. The strength of the magnetic field has to be strong enough in order to saturate the entire ribbon ferromagnetically across the ribbon width. This can be achieved in magnetic fields as low as a few hundred Oe. Such transverse field-annealing can be performed, for example, batch-wise either on toroidally wound cores or on pre-cut straight ribbon strips. Alternatively, and as disclosed in detail in U.S. Pat. No. 5,469,140, the annealing can be performed in a continuous mode by transporting the alloy ribbon from one reel to another reel through an oven in which a transverse saturating field is applied to the ribbon.
The change of magnetization by rotation and the associated magnetoelastic properties are primarily related to the fact that there is a uniaxial anisotropy axis perpendicular to the applied operational magnetic field. The anisotropy axis need not necessarily be in the ribbon plane like in the case of the transversely field annealed samples; the uniaxial anisotropy can also be caused by mechanisms other than field annealing. A typical situation is, for example, that the anisotropy is perpendicular to the ribbon plane. Such an anisotropy can arise again from magnetic field annealing but this time in a strong field oriented normal to the ribbon's plane, as taught by Gyorgy, in Metallic Glasses, 1978, Proc. ASM Seminar Sept. 1976 (American Society for Metals, Metals Park, ohio) ch. 11, pp 275-303, U.S. Pat. No. 4,268,325, Grimm et al., 1985, "Minimization of Eddy Current Losses in Metallic Glasses by Magnetic Field Heat Treatment", Proceedings of the SMM 7 conference in Blackpool (Wolfson Centre for Magnetics Technology, Cardiff) p. 332-336, de Wit et al., 1985 "Domain patterns and high-frequency magnetic properties of amorphous metal ribbons" J. Appl. Phys. vol 57, pp. 3560-3562 (1985), and Livingston et al., "Magnetic Domains in Amorphous Metal ribbons", J. Appl. Phys. vol. 57, pp 3555-3559 (1985), which hereafter will be referred to as perpendicular field-annealing. Other sources of such a perpendicular anisotropy can arise from the magnetostrictive coupling with internal mechanical stresses associated with the production process (see the aforementioned Livingston et al., "Magnetic Domains in Amorphous Metal ribbons" article and the aforementioned chapter by Fujimori in F. E. Luborsky (ed)) or e.g. induced by partial crystallization of the surface (Herzer G. "Surface Crystallization and Magnetic Properties in Amorphous Iron Rich Alloys", J. Magn. Magn. Mat., vol. 62, p. 143-151 (1986)).
When the magnetic easy axis is perpendicular to the ribbon plane, the large demagnetization factor requires very fine domain structures in order to reduce magnetostatic stray field energy (cf. Landau et al. in Electrodynamics of Continous Media, Pergamon, Oxford, England, ch 7. (1981)). Domain widths observed are typically 10 .mu.m or less and the visible domains are generally closure domains while ribbons with an anisotropy across the ribbon width exhibit wide transverse slab domains, typically about 100 .mu.m in width (as taught by the aforementioned Gyorgy article and the aforementioned de Wit et al. article, and Mermelstein, "A Magnetoelastic Metallic Glass Low-Frequency Magnetometer", IEEE Transactions on Magnetics, vol. 28, p. 36-56 (1992)).
One of the first examples for perpendicular field annealing was given in the aforementioned article by Gyorgy in which, for a Co-based amorphous alloy, the domain structure after said annealing treatment is compared with that obtained after a transverse field-anneal treatment and a longitudinal field anneal treatment, respectively. Gyorgy states that the domain structure of the perpendicularly annealed sample is typical for a uniaxial material with the easy axis normal to the surface.
The latter finding was confirmed in the aforementioned de Wit et al. article wherein two samples of a near-zero magnetostrictive amorphous Co-base alloy are compared, one having been transversely field-annealed in a field of 0.9 kOe and the other having been perpendicularly field annealed in a field of 15 kOe. de Wit et al. found that, as already mentioned above, in both cases the magnetization process is controlled by rotation which results in an essentially linear behavior of the magnetization with the applied field. The aforementioned Mermelstein article reaches a similar conclusion for a highly magnetostrictive amorphous Fe-based ribbon which was transversely and perpendicularly field-annealed, respectively, in a magnetic field of 8.8 kOe. Mermelstein posits that in both cases the magnetization process is controlled by rotation of the magnetization vector towards the applied field, and thus concludes it is sufficient to use a single model in order to describe the magnetic and magnetoelastic properties as well as the effect of eddy currents in both cases. Mermelstein's investigations were directed to a magnetoelastic field sensor using these samples and he concludes that both types of domain structures exhibit nominally equivalent noise baselines and that any differences in the sensor's sensitivity are only to be attributed to the differing anisotropy fields associated with dissimilarities in the heat treatment.
Still, as noted above de Wit et al. found that although essentially linear, the hysteresis loop of the perpendicularly annealed sample revealed a non-linear opening in its center region which is accompanied by enhanced eddy current losses, unlike the transversely annealed sample. This finding has been confirmed in the aforementioned Grimm et al., article which reports investigation of the perpendicular anisotropy in amorphous FeCo-and FeNi-based alloys induced by annealing in a magnetic field of 9 kOe oriented normal to the ribbon surface. Grimm et al. attribute this non-linearity to switching processes in the closure domains. Only in the case of the sample which had the highest magnetostriction (.lambda..sub.s.apprxeq.22 ppm) did they find a substantially linear magnetization loop with negligible hysteresis and considerably reduced eddy current losses. They found that in this case magnetostrictive interactions favor the closure domains to be oriented perpendicular to the applied field, which results in a less complex magnetization process within the closure domains. In contrast, the closure domain stripes are oriented parallel to the applied field for samples with lower magnetostriction constants (i.e. about 9 ppm in one example or a near-zero magnetostrictive sample), which results in the aforementioned non-linearity in the hysteresis loop's center region.
Comparable results also have been disclosed in the aforementioned U.S. Pat. No. 4,268,325 which describes annealed ring-laminated, toroidal cores assembled from punchouts from a 2 cm wide amorphous glassy Fe.sub.40 Ni.sub.40 B20 ribbon in a perpendicular field of 2 kOe and a circumferrential field of 1 Oe. According to this patent, the application of such a perpendicular field during annealing results in a sheet having an easy magnetic axis essentially normal to the sheet plane. The result was a relatively linear magnetization loop but again with a non-linear opening in its center region and enhanced AC losses. The aforementioned U.S. Pat. No. 4,268,325, moreover, teaches that it is advantageous to apply in a second annealing step a magnetic field normal to the direction of the first field in order to minimize AC hysteresis losses. Indeed the losses of the cited sample could be improved by subsequent annealings in a circumferrential field. This second annealing step increases the remanence, and thus the non-linearity, and led to a minimum at an enhanced remanence of about 3.5 kG where the hysteresis loop was substantially non-linear.
All these observations teach that no real benefit is associated with perpendicular field-annealing over transverse field-annealing. Indeed, transverse field-annealing seems to be clearly advantageous if a linear hysteresis loop and low eddy current losses are required for whatever application. Moreover, transverse field-annealing is much easier to conduct experimentally than perpendicular field-annealing due in part to the field strengths needed to saturate the ribbon ferromagnetically in the respective cases in order to obtain a uniform anisotropy. Owing to their magnetic softness, amorphous ribbons can be generally saturated ferromagnetically in internal magnetic fields of a few hundred Oersteds. The internal magnetic field in a sample with finite dimensions, however, is composed of the externally applied field and the demagnetizing field, which acts opposite to the applied field. While the demagnetizing field across the ribbon width is relatively small, the demagnetizing field normal to the ribbon plane is fairly large and, for a single ribbon, almost equals the component of the saturation magnetization normal to the ribbon plane. Accordingly, in the aforementioned U.S. Pat. No. 4,268,325 it is taught that the strength of the perpendicularly applied magnetic field preferably should be at least about 1.1 times the saturation induction at the annealing temperature. This is typically accomplished by a field strength of about 10 kOe or more as reported in the aforementioned papers relating to perpendicular field annealing. In comparison transverse field-annealing can be successfully done in considerably lower fields in excess of a few hundred Oe only. The aforementioned U.S. Pat. No. 5,469,140 as well as European Application 0 737 986, for example, teach that for transverse field-annealing a field strength in excess of 500 Oe or 800 Oe is enough to achieve saturation. Of course such a moderate field can be realized in a much easier and a more economic way than the high fields necessary for perpendicular annealing. Thus, lower magnetic fields allow a wider gap in the magnet, which facilitates the construction of the oven which has to be placed within this gap. If the field is produced by an electromagnet, moreover, the power consumption is reduced. For a yoke built of permanent magnets lower field strengths can be realized with less and/or cheaper magnets.